The present invention relates to an improved nonlinear element that makes utilization of tunnelling effects and to a bistable memory device formed by such a nonlinear element.
It has been known in the art that, even in such a situation that an insulating layer having a barrier level higher than the conductive level at which electrons can move is provided between two conductive layers, tunneling occurs so that electrons flow from one conductive layer to the other at a certain probability. If a voltage, applied between the conductive layers, is increased, then the possibility that electrons move likewise increases. As a result, the current flowing between the conductive layers increases. Further, if the voltage applied between the conductive layers goes up to above a certain voltage level, then the barrier level of the insulating layer lowers in effect, as a result of which diffusion current and FN tunnelling occur thereby increasing the current. This voltage-current characteristic that the current increases simply with the voltage is a basic characteristic exhibited in the usual conductor-insulator-conductor structure.
Nonlinear elements have been known in the art. A nonlinear element is one that has the nonlinear characteristic that the current does not increase simply with the voltage. Of various nonlinear elements a tunnel diode is a typical nonlinear element. The tunnel diode is described here. Both the impurity concentration of a p-type semiconductor layer and the impurity concentration of an n-type semiconductor on both sides of a semiconductor pn junction are made sufficiently high, to reduce the width of depletion layer. In this situation, a voltage in the forward direction is applied to the pn junction. The applied voltage is increased. When the applied voltage reaches a certain voltage level, the current reaches maximum value. Thereafter, if the applied voltage continues to increase, the current decreases (negative resistance). If the applied voltage is further increased, the current resumes increasing. In other words, the tunneling diode utilizes the phenomenon that tunnelling effects occur when the conduction band of the n-type semiconductor layer and the valence band of the p-type semiconductor layer are matched in energy level in a certain voltage range. Tunneling diodes find applications in bistable multivibrators and fast logic circuits because of their advantages such as fast switching characteristics.
However, there have been demands for faster elements for high-frequency signals of GHz or more, and for elements capable of being driven by a low voltage (several volts). Conventional tunnel diodes find it difficult to meet these requirements.
With a view to meeting the above noted requirements, an element is proposed which makes use of resonance tunnelling. For example, Japanese Patent Application (Pub. No. 3-148183) shows such a new element. In accordance with this application, an InAs oxide film, which acts as an insulating layer, is formed between InAs compound semiconductor layers. Quantum tunnelling by triangular potential, which occurs when the InAs substrates are matched in their quantum level, is utilized, to provide a semiconductor device having a nonlinear region in its voltage-current characteristic. Such a structure is implemented as follows. An InAs oxide film is formed on each InAs substrate and the InAs substrates are laminated together.
The above-described technique making use of resonance tunnelling resulting from a structure in which an insulating layer is just formed between compound semiconductor layers, however, has the following problems.
A resonance tunnelling phenomenon that occurs between quantum levels by the triangular potential of the compound semiconductor layers involves phonon scattering and therefore the probability that electrons move is as low as one resulting from simply applying a voltage to both sides of an insulating layer. The degree of nonlinearity exhibited in the voltage-current characteristic is extremely low, because of which the range of applications is limited.
The quantum level is likely to vary depending on the magnitude of applied voltage. If a slightly higher voltage is applied, this prevents a triangular potential from being formed therefore leading to the disappearance of the quantum levels. No resonance tunnelling occurs accordingly.
Although the use of compound semiconductors enables high-speed operations, they are expensive. Therefore, the application range is limited. Additionally, the fabrication process of the above-described technique may find it difficult to mount on the same semiconductor integrated circuit both nonlinear elements and transistors.